Iberulite fall and formation mechanism during a Sahara dust event in Switzerland in February 2021

Grobety, Bernard; Favreau, Philippe; Rausch, Juanita; Jaramillo, David; Steinbacher, Martin; Neururer, Christoph

doi:10.5194/ar-2026-6

Preprints

https://doi.org/10.5194/ar-2026-6

Preprints

04 Feb 2026

| 04 Feb 2026

Status: this preprint is currently under review for the journal AR.

Iberulite fall and formation mechanism during a Sahara dust event in Switzerland in February 2021

Bernard Grobety, Philippe Favreau, Juanita Rausch, David Jaramillo, Martin Steinbacher, and Christoph Neururer

Abstract. Sahara dust events can promote the formation of micrometer‑sized spherical mineral aggregates known as iberulites as they are usually found on the Iberian Peninsula. Iberulites have not, to our knowledge, been reported from Central Europe. Two formation mechanisms – in- and below scavenging – have been given. Strong iberulite fall (IFs) were observed in Western Switzerland in February 2021 when large amounts of dust were transported from the Moroccan-Algerian border towards Central Europe during a Saharan Dust Event (SDE). In contrast to the previous IFs observed on the Iberian Peninsula, this IF occurred under low-temperature conditions e.g. near the freezing point in the cloud and at the surface. The relative humidity in the first 1000 m below the clouds decreased from 70 to 40 %. and the comparison between particle-size distribution (PSD) within the iberulites and the dust PSD in the free atmosphere revealed a Greenfield gaprelative to the dust in the free atmosphere. The meteorological conditions, the microstructure of the iberulites, and the presence of a Greenfield gap point to below-cloud scavenging as the most likely formation mechanism. Further discoveries of iberulite in Swiss SDE samples suggest that the phenomenon is widespread and not only limited to regions close to the dust source.

Received: 29 Jan 2026 – Discussion started: 04 Feb 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Bernard Grobety, Philippe Favreau, Juanita Rausch, David Jaramillo, Martin Steinbacher, and Christoph Neururer

Status: open (until 05 Apr 2026)

Post a comment Subscribe to comment alert

RC1: 'Comment on ar-2026-6', Anonymous Referee #1, 06 Mar 2026 reply

The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2026-6/ar-2026-6-RC1-supplement.pdf
Reply

Citation: https://doi.org/10.5194/ar-2026-6-RC1
RC2:
'Comment on ar-2026-6', Anonymous Referee #2, 20 Mar 2026 reply
The submitted manuscript “Iberulite fall and formation mechanism during a Sahara dust event in Switzerland in February 2021,” reports on field measurements of iberulite dust particles, discusses formation/transport mechanisms, and suggest the breadth of the underlying phenomenon. Broadly, the manuscript text is well organized, and the data reported is novel. Given the growing relevance of atmospheric dust for human health applications, as well as climatology, I believe the manuscript is within the scope and impact the journal seeks. However, the current organization of evidence in the manuscript, particularly the figures, requires substantive revision. Therefore, I recommend it for publication only after major revision.
Major Comments
Some of the claims in the introduction are not detailed enough and seem more like bullet points or sentence fragments. For example, in Line 37, you simply write “Dust also affects climate and ecosystems.” There is no interpretation of what this means physically/chemically/biologically, what the cost is (monetary and/or health), and most importantly why the community should care. Please restructure the first paragraph to give more concrete examples of why desert dust transport is important. It is not necessary to make it much longer, but rather more centered on meaningful claims backed by the literature.

The second paragraph of the introduction is poorly organized. Please split into three paragraphs: (1) paragraph explaining what Iberulite is, and its relevance, (2) SDEs in Switzerland, monitoring infrastructure etc., (3) introducing the hypothesis, study design, and research goals.

In Methods section 2.1, some mention/citation/discussion of standards in dust collection is needed. Are the methods used considered “state-of-the-science”? Why are they different What implications does this have for study credibility?

Section 2.3 (“Scanning Electron (SEM) and Optical Polarization Microscopy (OPM) analyses”) of the methods is poorly organized, and difficult to understand the relevance to the study. I recommend 1) briefly describing the method with citations, 2) if any section is different from typical literature methods describe why and how it impacts the findings, 3) move all other technical details to a supplement/appendix (since they are not novel nor directly relevant). In this way, you can combine with section 2.4, for a single concise and readable analytical methods section.

Figure 1 is not appropriate for scientific journal articles. Figure 1 is a diagnostic plot automatically generated from HYSPLIT, it is both missing key information (e.g., axis labels on the line plot in the middle panel), and contains extraneous information (e.g., “Job ID”). Either extract the relevant data to combine with the suggested map of measurement locations (see Minor Comment #3), eliminate, or move to supplement (with a note that it is an automatically generated diagnostic plot).

Figure 2 is not appropriate for scientific journal articles. There is no legend presented, so it is impossible to interpret the plot. Readers cannot tell what units are used (if any), the geographic scale, or many other necessary components. Please remove. If you want to show the data, I recommend a simple time series of the dust product in the supplement.

Figure 3 is not appropriate for scientific journal articles. The resolution is bad – it is blurry. Although it at least has an appended legend, the figure is difficult to interpret and does not directly support the arguments of the paper as well as the trajectory analysis or analytical methods applied to samples do. I recommend it to be removed entirely.

Figure 4 requires significant revision. The figure resolution is bad, too blurry. Why are there breaks in the horizontal lines from y-axis in panel a? If the figures are sharing a y-axis, generally, the y-axis labels should be on the left-most figure, not the right figure. The green line in panel a should also be removed.

Figure 5 requires significant revision. I strongly encourage the authors to review the figure and illustration guidelines from the publisher. The colormap used is unscientific (i.e., biased in color intensity), and therefore misleading (see http://blogs.egu.eu/divisions/gd/2017/08/23/the-rainbow-colour-map/). The figure labels are not thoughtfully placed, and are distracting (e.g., “cloud base” has a dotted line running though it; “lower thick SD level” looks like it may be covering important data). I recommend extracting the underlying data, plotting anew, and carefully placing labels to ensure no overlaps.

Figure 9 only referenced once, and it is not essential to the arguments being made. Please remove.

Section 3.4 (“PSDs at JFJ and inside the iberulites”) does not flow well either internally or with respect to the rest of the paper. I recommend citing any routine calculation (e.g., equivalent volume diameter), moving any non-routine calculations to their own section of the methods section, and simply give the results with a concise description here.

Figure 11 is not appropriate for scientific journal articles . It looks like another diagnostic plot with overlapping figure captions. The text on the figure is also too small to easily read and grasp the meaning of the figure. I recommend moving to a supplement or appendix after fixing the text size and captions.

Figure 12 seems like the most important figure but requires significant revision. The it suffers from the same problems as the other figures: the text is unreadable, bad resolution, unclear units, seems automatically generated rather than extracted data. Please follow the feedback from the other figures and apply here.

Section 4.1 (“Dust Composition”) is just literature review, not needed in the Discussion. Remove it, or fold it into the introduction section.

Section 4.2 (“Iberulite formation and internal structure”) needs to be reorganized.
Line 302-303: what does it mean that the IF was “observed in person” – does this mean that there is some qualitative flag recorded? Please elaborate.

Lines 304-307 restate what has already been mentioned twice before, please make more concise.

Lines 315-324 muddle the arguments which follow by introducing yet another equation that is not explored, plotted, or constitutes a novel result.

Lines 326-337 should be their own paragraph, please try to refrain from introducing too many new concepts

Lines 338-340 is built on anecdotal evidence (“some iberulites striking car windshields were still wet”). Please use some of the abundant meteorological data in the paper to support this finding. Anecdotal evidence is notoriously unreliable; there could multiple other explanations for wet windshields.

Much of this section reads like an outline or informal note rather than a scientific journal submission. The parenthetical notes on lines 353, 357, and 359 as well as the general sentence structure is excessively informal or fragmented.

Pages 19-20 introduce many new concepts and literature review. Please condense considerably to only the most essential points regarding iberulite formation.

The last paragraph of the section (lines 397-403) is well-written and neatly summarizes many of the concepts necessary to understand the field observations. Please eliminate the extraneous concepts in the section unnecessary to understanding this paragraph.

The acknowledgement section seems to suggest that this manuscript was a rough draft or outline. Why are there question marks for some contributors in lines 426-427? Please give a very close reading before re-submission to ensure unnecessary figures are eliminated, sentence fragments are rectified, extraneous information removed, and data is presented in the correct section.

Minor Comments
At lines 34-37, you claim aerosol concentrations during these events often exceed air quality standards, it would be useful to the reader to first state what the standards are (Annual? Daily? Hourly?), what an example concentration looks like (and at what scale – hourly? Weekly sustained?)

You conclude your first paragraph with “dust mineralogical studies are scarce,” a reasonable statement, but support with a paper from 2010. Has there been any growth or decline in atmospheric mineralogical studies in Europe or elsewhere? A more recent review is warranted.

Line 65: “see below” – what does this refer to? Use figure numbers

There are many collection locations to keep track of in section 2.1. A map with these locations and their terrain either here or in a supplement would be useful.

Line 71: looks like two different sections called “2.1,” please make second “2.1” into 2.2

In Line 84, please cite the relevant peer-reviewed article on the satellite rather than sharing a link, and briefly describe its relevance.

The paragraphs in Section “Meteorological Data” gives too many abbreviations for unnecessary terms, e.g., “NOAA” or “ARL.” Simply cite the papers, and use the full form for terms used only a few times.

Figure 6 requires some revision. Again, the resolution is bad – too blurry. The figure title is irrelevant, please remove or make it more easily readable. Also consider removing the blue line, and simply keeping the uncertainty band, as it more convincingly tells the story.

Either remove Figure 7, or move to supplement/appendix.

Reply
Citation: https://doi.org/10.5194/ar-2026-6-RC2

Bernard Grobety, Philippe Favreau, Juanita Rausch, David Jaramillo, Martin Steinbacher, and Christoph Neururer

Viewed

Total article views: 220 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	BibTeX	EndNote
167	39	14	220	11	14

HTML: 167
PDF: 39
XML: 14
Total: 220
BibTeX: 11
EndNote: 14

Views and downloads (calculated since 04 Feb 2026)

Month	HTML	PDF	XML	Total
Feb 2026	102	26	10	138
Mar 2026	65	13	4	82

Cumulative views and downloads (calculated since 04 Feb 2026)

Month	HTML	PDF	XML	Total
Feb 2026	102	26	10	138
Mar 2026	65	13	4	82

Viewed (geographical distribution)

Total article views: 211 (including HTML, PDF, and XML) Thereof 211 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 21 Mar 2026

Short summary

The iberulite fall was, to our knowledge, the first reported in central Europe. Special was, that the fall was observed in-situ and that the meteorological conditions during the fall are well known. The meteorology, the particle size distribution inside the iberulites, and the presence of a Greenfield gap in the latter strongly corroborates below-cloud scavenging as probable formation mechanism.


Total:	0
HTML:	0
PDF:	0
XML:	0